1. Field of the Invention
The invention relates in general to electrical apparatus and, more particularly, to automatic transfer control devices for selectively energizing an electrical distribution system from a plurality of electrical power sources.
2. Description of the Prior Art
In supplying electrical power to industrial and commercial facilities, it is often desirable to provide alternate sources of electrical power to insure continuity of service. Sometimes these sources may comprise separate feeder circuits from the electric utility company. In other situations one or more diesel generators may be provided as alternate sources. Means must be provided to switch the distribution system between the alternate sources, and it is often desirable to provide this switching capability as an automatic function. Thus, if the primary power source should fail, the transfer control device will automatically switch the distribution system from the primary to the alternate source. In order to provide the desired features for each individual installation many options are often specified, including automatic retransfer when the primary source once again returns to normal, time delay before switching, interlocking to prevent the load from being connected on a transient basis to both sources at the same time, automatic startup of diesel generators, division of the load between the sources, and others.
In providing an automatic transfer control device for a specific application, it was usually necessary to engineer a custom design for each application, selecting various relays and components to provide the desired features. Prior art automatic transfer control devices have sometimes provided a certain degree of flexibility, but have often required auxiliary relays and components. In addition, prior art automatic control transfer devices employing electromechanical logic components have required substantial amounts of power. It woul be desirable to provide an automatic transfer control device having sufficient flexibility to handle a wide variety of transfer control applications including both two-breaker schemes and three-breaker schemes having two sources and two loads.
In addition, it is desirable to provide means for monitoring the frequency of the sources. Prior art devices have sometimes required extended periods of time to determine the frequency. It would be desirable to provide means for rapidly determining the frequency of the sources.
Prior art frequency monitors have sometimes provided return-to-normal limit values different from the alarm values, but these values were dependent on the alarm values. It would therefore be desirable to provide means for comparing the source frequency after an alarm with a return-to-normal limit value which is independent of the alarm value.
Since the automatic transfer control device is subject to momentary interruptions in control power, it is desirable to provide means for storing energy to power the device during these interruptions and to provide circuitry having minimum power requirements.
Power sources in industrial environments often have high noise components. It is therefore desirable to provide an automatic transfer control device having a high degree of noise immunity.